Various devices have heretofore been made for clamping an axially movable rod to hold it against movement relative to the clamp device and some of these clamp devices such as disclosed in U.S. Pat. Nos. 3,470,793; 3,575,087; 3,995,534; 4,463,481 and 5,302,062, disclose spring arrangements for normally urging the clamp device to one condition and fluid pressure operated means for moving the clamp device to a second condition. The rod clamp should be capable of clamping the piston rod in any extended condition of the rod. The length of the piston rod must be increased to accommodate the overall length of the clamp device, and it is accordingly desirable to minimize the overall length of the clamp device. In some installations, the fluid pressure available for actuating the clamp device is low, and difficulties have been encountered in providing a rod clamp device that can be reliably operated at low fluid pressures. Further, many prior rod clamp devices were expensive to manufacture and assemble.